Apparatus and System for Well Placement and Reservoir Characterization

ABSTRACT

A resistivity array having a modular design includes a transmitter module with at least one antenna, wherein the transmitter module has connectors on both ends adapted to connect with other downhole tools; and a receiver module with at least one antenna, wherein the transmitter module has connectors on both ends adapted to connect with other downhole tools; and wherein the transmitter module and the receiver module are spaced apart on a drill string and separated by at least one downhole tool. Each transmitter and receiver module may comprise at least one antenna coil with a magnetic moment orientation not limited to the tool longitudinal direction. A spacing between the transmitter and receiver module may be selected based on expected reservoir thickness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims the benefit, under 35 U.S.C. § 120,of U.S. application Ser. No. 11/160,533, filed on Jun. 28, 2005, whichclaims the benefit, under 35 U.S.C. § 119, of U.S. ProvisionalApplication Ser. No. 60/587,689, filed on Jul. 14, 2004.

BACKGROUND

1. Field of the Invention

This invention relates to the field of subsurface exploration and, moreparticularly, to techniques for determining subsurface parameters andwell placement. The invention has general application to the welllogging art, but the invention is particularly useful in logging whiledrilling (LWD), measurement-while-drilling (MWVD), and directionaldrilling (Geo-steering) applications.

2. Background Art

Electromagnetic (EM) logging tools have been employed in the field ofsubsurface exploration for many years. These logging tools orinstruments each have an elongated support equipped with antennas thatare operable as sources (transmitters) or sensors (receivers). Theantennas on these tools are generally formed as loops or coils ofconductive wires. In operation, a transmitter antenna is energized by analternating current to emit EM energy through the borehole fluid (“mud”)and into the surrounding formations. The emitted energy interacts withthe borehole and formation to produce signals that are detected andmeasured by one or more receiver antennas. The detected signals reflectthe interactions with the mud and the formation. The measurements arealso affected by mud filtrate invasion that changes the properties ofthe rock near the wellbore. By processing the detected signal data, alog or profile of the formation and/or borehole properties isdetermined.

The processing of the measured subsurface parameters is done through aprocess known as an inversion technique. Inversion processing generallyincludes making an initial estimate, or model, of the geometry of earthformations, and the properties of the formations, surrounding the welllogging instrument. The initial model parameters may be derived invarious ways known in the art. An expected logging instrument responseis calculated based on the initial model. The calculated response isthen compared with the measured response of the logging instrument.Differences between the calculated response and the measured responseare used to adjust the parameters of the initial model. The adjustedmodel is used to again calculate an expected response of the welllogging instrument. The expected response for the adjusted model iscompared with the measured instrument response, and any differencebetween them is used to again adjust the model. This process isgenerally repeated until the differences between the expected responseand the measured response fall below a pre-selected threshold. U.S. Pat.No. 6,594,584 describes modern inversion techniques and is incorporatedherein by reference in its entirety.

Well placement in real-time using resistivity measurements has been usedby the industry since the availability of LWD and MWD tools. Thisapplication is commonly known as geo-steering. In geosteering,estimation of the borehole position in real-time with respect to knowngeological markers is performed through correlation of resistivity logfeatures. Because of the typical close placement of the resistivitysensors of a LWD tool along the drill collar, only limited radialsensitivity is attained, thereby limiting the extent of the formationgeological model knowledge and estimation. Only with the introduction ofsensors with transmitter receiver distance in the tens of meters, adeeper radial sensitivity can be obtained.

Schlumberger's LWD Ultra Deep Resistivity (UDR) induction tool, withlarge transmitter receiver spacing in the tens of meters has beensuccessfully tested. One application of the tool has been to determinethe location of an oil-water contact (OWC) 7-11 m away from the wellpath. U.S. Pat. No. 6,188,222, titled “Method and Apparatus forMeasuring Resistivity of an Earth Formation” and issued to Seydoux etal., and U.S. patent application Ser. No. 10/707,985, titled “Systemsfor Deep Resistivity While Drilling for Proactive Geosteering” bySeydoux et al., provide further description of these tools and usethereof. The '222 patent and the '985 application are assigned to theassignee of the present invention and are incorporated by reference intheir entireties.

The LWD ultra deep resistivity basic tool configuration comprises twoindependent drilling subs (one transmitter and one receiver) that areplaced in a BHA among other drilling tools to allow largetransmitter-receiver spacing. The basic measurements obtained with thistool consist of induction amplitudes at various frequencies, in order toallow detection of various formation layer boundaries with resistivitycontrasts having a wide range of resistivities. The measurements areused to invert for an optimum parameterized formation model that givesthe best fit between actual tool measurements and the expectedmeasurements for the tool in such a formation model.

FIG. 1 shows an example of an MWD tool in use. In the configuration ofFIG. 1, a drill string 10 generally includes kelly 8, lengths of drillpipe 11, and drill collars 12, as shown suspended in a borehole 13 thatis drilled through an earth formation 9. A drill bit 14 at the lower endof the drill string is rotated by the drive shaft 15 connected to thedrilling motor assembly 16. This motor is powered by drilling mudcirculated down through the bore of the drill string 10 and back up tothe surface via the borehole annulus 13 a. The motor assembly 16includes a power section (rotor/stator or turbine) that drives the drillbit and a bent housing 17 that establishes a small bend angle at itsbend point which causes the borehole 13 to curve in the plane of thebend angle and gradually establish a new borehole inclination. The benthousing can be a fixed angle device, or it can be a surface adjustableassembly. The bent housing also can be a downhole adjustable assembly asdisclosed in U.S. Pat. No. 5,117,927, which is incorporated herein byreference. Alternately, the motor assembly 16 can include a straighthousing and can be used in association with a bent sub well known in theart and located in the drill string above the motor assembly 16 toprovide the bend angle.

Above the motor assembly 16 in this drill string is a conventional MWDtool 18, which has sensors that measure various downhole parameters.Drilling, drill bit and earth formation parameters are the types ofparameters measured by the MWD system. Drilling parameters include thedirection and inclination of the BHA. Drill bit parameters includemeasurements such as weight on bit (WOB), torque on bit and drive shaftspeed. Formation parameters include measurements such as natural gammaray emission, resistivity of the formations, and other parameters thatcharacterize the formation. Measurement signals, representative of thesedownhole parameters and characteristics, taken by the MWD system aretelemetered to the surface by transmitters in real time or recorded inmemory for use when the BHA is brought back to the surface.

Although the prior art deep-reading resistivity tools (such as UDR)proved to be invaluable in geosteering applications, there remains aneed for further improved deep-reading resistivity tools that can beused in geosteering and/or other applications.

SUMMARY

One aspect of the invention relates to a resistivity array having amodular design. A resistivity array in accordance with one embodiment ofthe invention includes a transmitter module with at least one antenna,wherein the transmitter module has connectors on both ends adapted toconnect with other downhole tools; and a receiver module with at leastone antenna, wherein the transmitter module has connectors on both endsadapted to connect with other downhole tools; and wherein thetransmitter module and the receiver module are spaced apart on a drillstring and separated by at least one downhole tool. Each transmitter andreceiver module may comprise at least one antenna coil with a magneticmoment orientation not limited to the tool longitudinal direction. Inthe case of more than one antenna, all antennas orientation vectors maybe linearly independent. A set of vectors are linearly independent ifand only if the matrix constructed from concatenating horizontally thevector's component has a rank equal to the number of vectors.

Another aspect of the invention relates to resistivity tools. Aresistivity tool in accordance with one embodiment of the inventionincludes a tool body adapted to move in a borehole; and at least threemodules (subs) disposed on the tool body, wherein the at least threemodules are not equally spaced along a longitudinal axis of the toolbody, such that a combination of the at least three modules comprises aresistivity array of different spacings.

Another aspect of the invention relate to resistivity tools. Aresistivity tool in accordance with one embodiment of the inventionincludes a tool body adapted to move in a borehole; a resistivity sensordisposed on the tool body and comprising a plurality of modules formingat least one array; and an additional antenna disposed on the tool bodyand spaced apart from the resistivity sensor along a longitudinal axisof the tool body, wherein the additional module and one of the pluralityof module in the resistivity sensor form an array having a spacinggreater than about 90 feet.

Another aspect of the invention relates to logging-while-drilling tools.A logging-while-drilling tool in accordance with one embodiment of theinvention includes a drill bit disposed at one end of a drill string; afirst module disposed on the drill string proximate the drill bit or inthe drill bit, and at least one additional module disposed on the drillstring, and spaced apart from the first module, wherein the first modulehas at least one antenna with magnetic moment orientation not limited tothe longitudinal direction, and wherein the at least one additionalmodule comprises three antennas whose magnetic moment orientations arelinearly independent.

Another aspect of the invention relates to methods for formationresistivity measurements. A method for formation resistivitymeasurements in accordance with one embodiment of the invention includestransmitting electromagnetic energy into a formation using a transmitterantenna in a resistivity array, wherein the transmitting is performedwith a plurality of frequencies according to a selected pulse scheme;and detecting, for each of the plurality of frequencies, a signalinduced in a receiver antenna spaced apart from the transmitter antennain the resistivity array.

Another aspect of the invention relates to methods for designing aresistivity array. A method for designing a resistivity array inaccordance with one embodiment of the invention includes estimating athickness of a reservoir; and disposing a transmitter and a receiver ona drill string such that a spacing between the transmitter and thereceiver is no less than the estimated thickness of the reservoir.

Other aspects of the invention will be apparent from the followingdescription and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a prior art drilling rig and drill string that can be usedwith one embodiment of the invention.

FIG. 2 shows a resistivity array in accordance with one embodiment ofthe present invention.

FIG. 3 shows a resistivity array in accordance with another embodimentof the present invention.

FIG. 4 shows examples of depth of investigation for a 10 kHz amplitudemeasurement obtained with various transmitter-receiver distances inaccordance with one embodiment of the present invention.

FIG. 5 shows a resistivity array in accordance with one embodiment ofthe present invention.

FIG. 6 shows a resistivity array in accordance with one embodiment ofthe present invention.

FIGS. 7A and 7B show amplitude responses of conventional prior artresistivity arrays.

FIGS. 7C and 7D show amplitude responses of resistivity arrays inaccordance with one embodiment of the present invention.

FIG. 8 shows a sequencing method in accordance with one embodiment ofthe present invention.

FIG. 9 shows a resistivity array in accordance with one embodiment ofthe present invention.

FIG. 10 shows an antenna module in accordance with one embodiment of thepresent invention.

FIGS. 11A-11F show various measurements for a planar boundary withresistivity contrast according to one embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to resistivity arrays havingimproved properties. Some embodiments of the invention relate to methodsof using these tools in formation evaluation. Embodiments of theinvention may permit inversion for more complicated formation models(i.e., formation model with more parameters) and/or may improve therobustness of resistivity measurement inversion (uncertainty reduction).Some embodiments of the invention may increase the flexibility offormation resistivity evaluation by providing more measurements, each ofwhich may have different responses to different formation models.

Some embodiments of the invention provide resistivity arrays having amodular design. The modular design facilitates setting up different toolconfigurations for different measurement requirements. For example, byextending the number of transmitter, receiver combinations (for example,one embodiment with four transmitters and one receiver, forming fourtransmitter-receiver arrays), more depths of investigation can beobtained.

Some embodiments of the invention may include antennas that can functionas a transceiver (i.e., as a transmitter and a receiver). This furtherprovides tool configuration flexibility. In this implementation, for thesame number of modules, a greater number of transmitter, receivercombinations can be achieved. Also, symmetrization of directionalmeasurement can be achieved, without extending the length of the tool ina manner similar to the published U.S. Patent Application No.2003/0085707 A1, by Minerbo et al.

Some embodiments of the invention relate to tools having a transmittersub at a great distance from the receiver (e.g., >90 ft) to allowselective sensitivity to reservoir complexity. Such an embodiment mayhave an independently powered transmitter sub placed outside (far awayfrom) a conventional bottom hole assembly.

Some embodiments of the invention relate to placement of a transmitterat or inside the drill bit, or very close to the drill bit, forlook-ahead capability. Such an embodiment may have an independentlypowered system and data communication capability.

Some embodiments of the invention relate to having at least one modulelocated in a separate well or borehole

Some embodiments of the invention relate to methods of formationresistivity evaluation using measurement frequencies tailored to theexpected formation. The frequency range, for example, may be up to 200KHz.

Some embodiments of the invention related to combining modules of theinvention with existing LWD resistivity arrays.

Some embodiments of the invention relate to coil designs that havemultiple windings to permit the use of the same antenna for a wide rangeof frequencies. The multiple windings may be connected in series orparallel.

Some embodiments of the invention related to extension of the amplitudemeasurement to phase, relative phase and amplitude as well as phaseshift and attenuation (propagation) that requires a sub to include tworeceiver antennas with relatively long spacing in the ten feet range.

Some embodiments of the invention relate to implementation ofdirectional antennas (co-located or in close proximity) with or withoutmetallic shields.

Tool Modularity

Some embodiments of the invention relate to resistivity arrays havingmodular designs. As used herein, a “resistivity array” is aconfiguration that includes at least one receiver module and at leastone transmitter module attached at different locations on a drillstring. The modular design allows the transmitter and receiver antennasto be placed at various locations within a BHA, or at locations in thedrill string above the BHA. For example, FIG. 2 shows a resistivityarray including four transmitter modules 21, 22, 23, 24 and one receivermodule 25 placed among other LWD or MWD tools 27, 28, 29, 30 in a BHA.By inserting transmitter and/or receiver modules at different locationson a standard BHA, as shown in FIG. 2, or a drill string, specificdepths of investigation can be implemented to optimize the formationmodel inversion process that uses such deep resistivity measurements.For example, in one embodiment, transmitter module 21 may be about 90 to100 feet from receiver module 25. In addition, one or more module may beplaced in a nearby borehole to provide a large spacing array.

The above-mentioned '985 application discloses an ultra-deep resistivityarray that may include transmitter and receiver modules. The '985application discusses the relationship between depth of investigation(“DOI”) and the spacing between a transmitter and a correspondingreceiver antenna, the relationship being that greater spacing results ina corresponding increase in DOI. The present inventors have found thatthe relationship holds true; however, increasing the spacing complicatesthe ability for a receiver to pickup and couple the signals from atransmitter. Embodiments of the present invention may use a tri-axialantenna in a transmitter or receiver module, wherein the tri-axialantenna module has three antennas having magnetic moments in threedifferent directions. The tri-axial antenna module will ensure that atleast some of the transverse components of the tri-axial antenna canform substantial coupling with the transverse component of acorresponding transmitter or receiver. The use of a tri-axial antennatransceiver (or receiver) is advantageous because when the drill stringis made up, it would be difficult to ensure that a single antennatransmitter will align with a single antenna receiver, with thatdifficulty increasing as the spacing increases. In contrast, thetri-axial antenna transceiver (or receiver) will always have a componentsubstantially aligned with the magnetic moment of a correspondingreceiver (or transceiver) in the resistivity array. In addition,tri-axial allows the determination of formation characteristics such asdip angle, anisotropy, shoulder bed effects.

FIG. 4 shows examples of depth of investigation for a 10 kHz amplitudemeasurement obtained with transmitter-receiver distances of 10, 30, 60and 90 ft in the presence of a boundary with resistivity contrast of 1to 10 ohms. The drill string (hence the resistivity array) is assumedparallel to the boundary and at various distances away from theboundary. As shown in FIG. 4, the 10 ft array is not very sensitive tothe boundary; it shows only a slight magnitude changes in the vicinityof the boundary. The 30 ft array is more sensitive, showing a distincttransition at the boundary. The 60 ft array is even more sensitive; itshows very pronounced resistivity transition around the boundary. Atthis transmitter-receiver spacing, the signal magnitude starts to changeat about 20-40 ft away from the boundary. With the 90 ft array, thesignal magnitude change is even more profound. It is apparent thatcombination of different depths of investigation allows differentiationsof geological formation at different radial distance. The modular designmakes it easy to configure the tools for different array spacing.Further, the use of one or more tri-axial antennas as transmittersand/or receivers increases the spacing that may be achieved, whichprovides a corresponding increase in DOI.

Modular Subs as Transceivers

Some embodiments of the invention relate to resistivity array designshaving transceiver antennas. In these tools, the antennas are notdesigned as separate transmitters or receivers. Instead, the sameantenna can function as either a transmitter or a receiver. Suchenhancement, besides being economically advantageous, allows more depthof investigation for the same number of subs, as illustrated in FIG. 3.

FIG. 3 shows a tool assembly 40 having three subs 41, 42, 43 that formtwo arrays with spacing of D and Dx2. Because the antennas 41 and 43 canfunction as a transmitter or a receiver, a third array having a spacingof Dx3 is also available with this tool configuration. Moreover, withthe transceiver antennas, directional measurements can also be performedwithout having to have both transmitter and receiver belonging to acommon downhole tool. For example, a set of symmetrized measurements maybe obtained first with antenna 41 as the transmitter and antenna 43 asthe receiver, then with antenna 43 as the transmitter and antenna 41 asthe receiver.

Remote Subs as Transmitter/Transceivers

Some embodiments of the invention relate to tools having antenna subsplaced far from other BHA tools (e.g., the receivers or transmitters).Wells often have curves and bends that limit the practical length of aBHA. Thus, conventional resistivity tools cannot have transmitters andreceivers spaced farther than the practical length limit of the BHA(about 150 feet). Such tools cannot provide the depth of investigationthat might be needed when placing a well path within a reservoir with athickness that exceeds the maximum practical length of a standarddrilling tool assembly.

FIG. 5 shows a resistivity array incorporating a remote sub inaccordance with one embodiment of the invention. As shown, theresistivity array includes a conventional UDR 51 in the BHA. The UDRincludes three antennas (transmitters, receivers, or transceivers) 52,53, 54. Further up the drill string, the resistivity array also includesa remote module 55, which includes a transmitter, a receiver, or atransceiver. The antenna in the remote module 55 may be used with any ofthe antennas 52, 53, 54 to form an array having a large spacing. Byusing a remote module 55 with other conventional resistivity tools inthe BHA, transmitter-receiver distances (i.e., array spacing) can be setto any desired distance. The remote module 55 may be independentlypowered. Furthermore, the remote module 55 may be operated by wirelesstelemetry, for example. In one embodiment, one or more drill collars 63may be located between the remote module 55 and one or more of theantennas 52, 53, 54.

The location of the remote module 55 may be selected to be on the orderof (or greater than) the reservoir thickness. Having an array spacing onthe order of (or greater than) the reservoir thickness can providedistinct advantages that are otherwise unavailable to conventionalresistivity tools.

For example, FIGS. 7C and 7D show that the amplitude responses of thelong array (the spacing of which is on the order of the bed thickness,130 ft) are much simpler and clearly indicate where the bed boundariesare. The responses of this extra long array (especially at lowfrequencies) are not sensitive to the reservoir internal complexity. Incontrast, as shown in FIGS. 7A and 7B, the amplitude responses ofconventional prior art resistivity arrays (the spacing of which aresmaller than the bed thickness, 130 ft) are more sensitive toresistivity variations within the bed, but less sensitive to bedboundaries. Results from FIGS. 7A-7D show that sensor distances (arrayspacing) and operational frequencies may be advantageously selectedbased on the properties of the reservoir being drilled, for example, theexpected bed thickness or the ratio of the lowest reservoir layerresistivity and the resistivity of the cap and reservoir bottom.

Look-Ahead with Subs at the Bit

Some embodiments of the invention relate to resistivity tools havinglook-ahead ability. In accordance with embodiments of the invention, asub may be placed proximate the drill bit in a way similar to thatdescribed in U.S. Pat. No. 6,057,784 issued to Schaff et al., andassigned to the assignee of the present invention. That patent isincorporated herein by reference in its entirety. In addition, anantenna can also be placed on a rotary steerable tool or directly insidea bit. By placing a transceiver at the bit, the resistivity measurepoint taken at the mid-distance between each transmitter/receiver pairis moved closer to the bit, thus allowing faster reaction time whiledrilling. This ability allows earlier real-time action to be taken toplace the well based on geological events. Moreover, look-ahead of thebit is also possible by using the tool response tail that extends beyonda resistivity sensor pair.

FIG. 6 shows one example of a resistivity array in accordance with oneembodiment of the invention. As shown, the resistivity tool 70 comprisesa drill bit 14 at one end of the drill string. An antenna 73 (which maybe a transmitter or a receiver antenna) is disposed on the drill stringproximate the drill bit 14. In addition, the resistivity array includesa UDR 51 having three transceiver modules 52, 53, 34, which can functionas receivers or transmitters. While three transceiver modules are shownin this example, one of ordinary skill in the art would appreciate thatsuch a tool may have more or less transceiver modules. Further, receiveror transmitter modules may replace one or more of the transceivermodules. In one embodiment, antenna 73 may be a component of drill bit14.

In accordance with some embodiments of the invention, the near-bitantenna 73 has a non-longitudinal magnetic moment, i.e., the magneticmoment of the antenna 73 is not in a direction parallel with the drillstring axis. The non-longitudinal magnetic moment of the antenna 73ensures that the antenna 73 has a component of the magnetic moment inthe transverse direction (i.e., the direction perpendicular to the drillstring axis). In addition, at least one of the transceiver modules(e.g., 52, 53, 54) comprises a tri-axial antenna, in which threeantennas have magnetic moments in three different orientations. In somecases, the tri-axial antennas may have magnetic moments in threeorthogonal orientations. The tri-axial antenna module will ensure thatat least some of the transverse components of the tri-axial antenna canform substantial coupling with the transverse component of the near-bitantenna 73. The near-bit antenna 73 may be a transmitter, receiver, or atransceiver. In general, it is preferable for the near-bit antenna 73 tobe a transmitter because a receiver antenna may see higher electricalnoise from increase vibration and shock or from a possible presence of ahigh power rotary steerable tool. As a result, the motor assembly 16,which may include powered steering components, can disrupt a receiverantenna.

Multi-Frequency Measurement

Some embodiments of the invention relate to tools and methods that usemulti-frequencies for resistivity measurements. In accordance withembodiments of the invention, frequencies may be selected to moreefficiently cover the frequency spectrum in order to improve theinversion accuracy and flexibility of deep resistivity measurements. Forexample, in accordance with some embodiments of the invention,measurements may be acquired with a distribution of 3 or morefrequencies per decade. These frequencies can be set or automaticallyselected, either before drilling or while drilling, to provide optimalformation inversion. The combination of transmitter receiver distancewith frequency is integral in the determination of reservoir outerboundaries with complex internal layer. The combination of antennaspacing and frequency are preferably selected to respect the followingequation for maximum sensitivity.

Let's define propagation coefficient k as: k²=εμω²+iσμω, where ε is theelectromagnetic permittivity, μ electromagnetic permeability, σconductivity, and ω the angular frequency. If L represents theTransmitter-Receiver spacing, then we want: |k|,Lε[0.1;10]. Thefrequencies are preferably chosen to meet this criterion.

The multi-frequency measurements can be efficiently performed using anyimplementation scheme known in the art. For example, FIG. 8 shows anexample of a resistivity measurement sequence for multi-frequencymeasurement. In the scheme shown in FIG. 8, all TX pulses are assumed tohave a controlled amplitude. Furthermore, one of ordinary skill in theart would appreciate that in the pulse scheme, as shown in FIG. 8, asingle pulse may be implemented to carry two or more frequencies. Signalmeasurements may be performed by measuring the true voltages as sensedby the receivers. Alternatively, the signals may be measured asdifferential signals between a pair of pulses of different frequencies.

Combination of Subs with Existing LWD Tools

Some embodiments of the invention relate to resistivity arrays havingremote subs, as described above, with other conventional resistivitytools. For example, FIG. 9 shows a tool including two remote subtransmitters, 55A and 55B, and a conventional LWD resistivity tool thatmay function as receivers for the remote transmitter modules to providearrays with spacing much longer than what is possible with conventionalresistivity arrays. One of ordinary skill in the art would appreciatethat any conventional resistivity tool having one or more antennas forreceiving resistivity signals may be used in combination with remote subtransmitters as disclosed herein. The option of running transmittermodules in combination with an existing “shallow” LWD tool (using theirresistivity antennas as deep resistivity receivers) allows assetrationalization and integrated measurement capabilities.

Multi-Winding Antenna

Some embodiments of the invention relate to antennas that may be usedefficiently in a wide frequency range. When an antenna is used totransmit a resistivity signal at a certain frequency, the antenna ismost efficient when the frequency is below the self-resonance frequencyof the antenna. Therefore, when a particular antenna is used in a widefrequency range, the antenna may not be efficient in certain frequencyranges. For example, to transmit at the highest frequency, the number ofturns in the antenna should be low enough to be below the coil selfresonance. On the other hand, to be optimum in transmission at a lowerfrequency, the number of turns needs to be increased. As a result,conventional antennas often have windings that represent a compromisefor the intended operational frequency range.

In accordance with some embodiments of the invention, an antenna mayhave several layers of windings; each of the layers may be either wiredin parallel for high frequency or in series for a lower frequency toefficiently balance the impedance load of the antenna when driven with aconstant voltage. The switching between serial and parallelconfigurations may be controlled electronically.

FIG. 10 shows an exemplary antenna in accordance with one embodiment ofthe invention. Coil layers 101A-101C, in this example, are eitherconnected in series to maximize the number of turns in the transmissionat low frequency (for example, around 1 kHz range), or are connected inparallel for the higher frequency range (for example, 100 kHz). The coillayers 101A-101C are wrapped around a mandrel 102. One of ordinary skillin the art would appreciate that several layers of coils may be used inan antenna to provide finer tuning of the performance of the antenna.

Extension to Other Resistivity Measurement Techniques

Conventional deep resistivity measurements, such as that disclosed inU.S. Pat. No. 6,188,222, are based on induction mechanism and measuressignal amplitudes, not phase or phase shifts or attenuations. Someembodiments of the invention relate to deep resistivity measurementsbased on propagation mechanism and measure phase shifts and attenuations(i.e., differential measurements), by introducing an extra receiverantenna with a spacing between the receiver pair on the order of 5 to 10feet, which is significantly longer than the receiver pair spacing(typically limited to less than a foot) in a conventional propagationtool. The longer spacing between the receiver pair is desirable becauseof the lower frequencies used for deep EM measurement (1 to 200 kHz). Aspacing between the receiver pairs on the order of 5 to 10 feet wouldensure that the minimum phase shift that can be measured stays in the0.1 degree range.

In addition to using a receiver pair, differential measurements in phaseand amplitude (i.e., phase shifts and attenuations) may also beperformed with a proper pulse scheme, such as that shown in FIG. 8. Themeasurement scheme shown in FIG. 8 may use one (or more) of thetransmitted pulses at a specific frequency as a time reference. Assuminga constant phase reference (or time difference) between pulses in thepulse train (this time difference can also be measured and communicatedto the receiver via wireless telemetry), the phase reference (or timedifference) for the received pulse trains can be determined with respectto the reference pulse.

The same technique (using the amplitude of a reference pulse forcomparison) can also be applied to the amplitude ratio between eachpulse in the pulse train and the reference pulse. In this case, theamplitude ratio at the transmitter may be kept constant or measured. Thedifference technique in pulse time of arrival and amplitude ratioreduces the dependence of the measurement on an accurate air calibrationas needed for the amplitude measurement.

As an example, FIGS. 11A-11F show various measurements for a planarboundary with resistivity contrast of 1 and 100 ohms, using a toolhaving a transmitter-receiver spacing of 70 feet. This tool hastransmitter and receiver antennas that have longitudinal magneticmoments (i.e., magnetic moments in a direction parallel with thelongitudinal axis of the tool).

FIG. 11A and FIG. 11B show amplitude measurements and relative amplitudemeasurements, respectively, at various frequencies. In FIG. 11B, therelative amplitude measurements are with respect to the amplitudemeasurement at 2 KHz. FIG. 11C and FIG. 11D show phase measurements andrelative phase measurements, respectively, at various frequencies. InFIG. 11D, the relative phase measurements are with respect to the phasemeasurement at 2 KHz.

FIG. 11E and FIG. 11F show phase shift measurements and attenuations,respectively, at various frequencies, as measured with a pair receivershaving an 8 feet spacing. With such a spacing, significant variations inPhase Shift and Attenuation can be readily observed. Both measurementprovide another set of measurements with a different sensitivityallowing more independent measurements to be used during the inversionprocess.

Some embodiments of the invention relate to geo-steering. A method ofgeo-steering in accordance with embodiments of the invention may use anyresistivity array described above and/or using a measurement methoddescribed above (e.g., multi-frequency measurements, use of a pulseschemes, etc.).

All measurements with the above-described embodiments of the inventioncan be extended to directional measurements. Directional measurementsallow further sensitivity to the boundaries and will improve theinversion process accordingly. In some embodiments, the antenna(s) wouldhave a transverse magnetic dipole combined with a normal “axial” antennato provide both measurements from the same antenna. In a tri-axialantenna, as discussed above, one portion may be aligned with the axis ofthe BRA, while the other two portions are at angles relative thereto.Conventional shields can also be implemented with embodiments of theinvention as desired. It will be appreciated that the antennas (andrelated electronics) of the embodiments of the invention may beimplemented using one of many well-known antenna designs and packagingschemes. For example, the logging apparatus described in U.S. Pat. No.6,188,222 may be used to implement the present invention.

While the above description uses logging-while-drilling tools toillustrate various embodiments of the invention, a tool of the inventionis not limited by any particular mode of conveyance. Therefore, a toolof the invention may be used in, for example, logging-while-drilling,logging-while-tripping, coil drilling, logging through the bit, linerdrilling, casing drilling operations.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1.-37. (canceled)
 38. A modular downhole apparatus to determine aformation property, the apparatus being incorporated into a drill stringcomprising one or more downhole tools and drill pipe, the drill pipebeing of the same or various lengths, the modular downhole apparatuscomprising: a first module comprising a first drill collar having one ormore antennas mounted thereon or therein, wherein the first module hasconnectors on both ends and is removeably connected to the drill string;and a second module comprising a second drill collar having one or moreantennas mounted thereon or therein, wherein the second module hasconnectors on both ends and is removeably connected to the drill string;wherein a first dipole moment of one of the one or more antennas of thefirst module intersects a first module longitudinal axis at a firstangle; wherein a second dipole moment of one of the one or more antennasof the second module intersects a second module longitudinal axis at asecond angle; and wherein the first and second angles are equal.
 39. Themodular downhole apparatus of claim 38, wherein the first and secondangles are non-longitudinal.
 40. The modular downhole apparatus of claim39, wherein the first and second dipole moments are azimuthally offsetfrom one another.
 41. The modular downhole apparatus of claim 38,wherein one of the modules has an antenna having a third dipole momentthat intersects the longitudinal axis of that module at a third anglethat is not equal to the first angle.
 42. The modular downhole apparatusof claim 38, wherein at least one of the modules comprises aconventional resistivity tool.
 43. The modular downhole apparatus ofclaim 38, wherein one or more of the one or more antennas in one or bothof the modules comprises a transceiver.
 44. The modular downholeapparatus of claim 38, wherein one or more of the one or more antennasin one of the modules transmits a signal and one or more of the one ormore antennas in the other module receives the signal.
 45. The modulardownhole apparatus of claim 38, wherein the one or more antennas of oneor both of the modules comprise transmitter antennas and receiverantennas.
 46. The modular downhole apparatus of claim 38, wherein one ormore of the one or more antennas of one of the modules is disposedproximate to or within a drill bit.
 47. The modular downhole apparatusof claim 38, wherein one of the modules includes a drill bit.
 48. Themodular downhole apparatus of claim 38, wherein one of the modules ispart of a logging-while-drilling resistivity tool.
 49. The modulardownhole apparatus of claim 38, further comprising one or moreadditional modules, each additional module having one or more antennas,wherein each additional module has connectors on both ends and isremoveably connected to the drill string.
 50. The modular downholeapparatus of claim 38, wherein one or more of the one or more antennasin one or both of the modules comprises a solenoid coil approximating amagnetic dipole.
 51. The modular downhole apparatus of claim 38, whereinthe one or more antennas on the first module comprise a plurality ofantennas having dipole moments that intersect the first modulelongitudinal axis at the first angle, and the one or more antennas onthe second module comprise a plurality of antennas having dipole momentsthat intersect the second module longitudinal axis at the second angle.52. The modular downhole apparatus of claim 38, wherein the formationproperty is one or more of a horizontal resistivity, a verticalresistivity, a formation resistivity, a formation factor, a fluidsaturation, a dip angle, a dip azimuth angle, a porosity, a distance tobed boundary, a formation conductivity tensor, or a permeability.
 53. Amodular downhole apparatus to determine a formation property, theapparatus being incorporated into a drill string comprising one or moredownhole tools and drill pipe, the drill pipe being of the same orvarious lengths, the modular downhole apparatus comprising: a firstmodule comprising a first drill collar having one or more antennasmounted thereon or therein, wherein the first module has connectors onboth ends and is removeably connected to the drill string; and a secondmodule comprising a second drill collar having one or more antennasmounted thereon or therein, wherein the second module has connectors onboth ends and is removeably connected to the drill string; wherein afirst dipole moment of one of the one or more antennas of the firstmodule intersects a first module longitudinal axis at a first angle;wherein a second dipole moment of one of the one or more antennas of thesecond module intersects a second module longitudinal axis at a secondangle; and wherein the first and second angles are unequal.
 54. Themodular downhole apparatus of claim 53, wherein the first and secondangles are non-longitudinal.
 55. The modular downhole apparatus of claim54, wherein the first and second dipole moments are azimuthally offsetfrom one another.
 56. The modular downhole apparatus of claim 53,wherein one of the modules has an antenna having a third dipole momentthat intersects the longitudinal axis of that module at a third anglethat is equal to neither the first angle nor the second angle.
 57. Themodular downhole apparatus of claim 53, wherein at least one of themodules comprises a conventional resistivity tool.
 58. The modulardownhole apparatus of claim 53, wherein one or more of the one or moreantennas in one or both of the modules comprises a transceiver.
 59. Themodular downhole apparatus of claim 53, wherein one or more of the oneor more antennas in one of the modules transmits a signal and one ormore of the one or more antennas in the other module receives thesignal.
 60. The modular downhole apparatus of claim 53, wherein the oneor more antennas of one or both of the modules comprise transmitterantennas and receiver antennas.
 61. The modular downhole apparatus ofclaim 53, wherein one or more of the one or more antennas of one of themodules is disposed proximate to or within a drill bit.
 62. The modulardownhole apparatus of claim 53, wherein one of the modules includes adrill bit.
 63. The modular downhole apparatus of claim 53, wherein oneof the modules is part of a logging-while-drilling resistivity tool. 64.The modular downhole apparatus of claim 53, further comprising one ormore additional modules, each additional module having one or moreantennas, wherein each additional module has connectors on both ends andis removeably connected to the drill string.
 65. The modular downholeapparatus of claim 53, wherein one or more of the one or more antennasin one or both of the modules comprises a solenoid coil approximating amagnetic dipole.
 66. A method to determine a formation property, themethod comprising: providing a modular apparatus incorporated into adrill string comprising one or more downhole tools and drill pipe, thedrill pipe being of the same or various lengths; the modular apparatuscomprising a first module comprising a first drill collar having one ormore antennas mounted thereon or therein, wherein the first module hasconnectors on both ends and is removeably connected to the drill string;a second module comprising a second drill collar having one or moreantennas mounted thereon or therein, wherein the second module hasconnectors on both ends and is removeably connected to the drill string;wherein a first dipole moment of one of the one or more antennas of thefirst module intersects a first module longitudinal axis at a firstangle; a second dipole moment of one of the one or more antennas of thesecond module intersects a second module longitudinal axis at a secondangle; and the first and second angles are equal; using the first andsecond modules to make measurements; and using the measurements todetermine the formation property.
 67. The method of claim 66, furthercomprising using the formation property to steer a drill bit.
 68. Amethod to determine a formation property, the method comprising:providing a modular apparatus incorporated into a drill stringcomprising one or more downhole tools and drill pipe, the drill pipebeing of the same or various lengths; the modular apparatus comprising afirst module comprising a first drill collar having one or more antennasmounted thereon or therein, wherein the first module has connectors onboth ends and is removeably connected to the drill string; a secondmodule comprising a second drill collar having one or more antennasmounted thereon or therein, wherein the second module has connectors onboth ends and is removeably connected to the drill string; wherein afirst dipole moment of one of the one or more antennas of the firstmodule intersects a first module longitudinal axis at a first angle; asecond dipole moment of one of the one or more antennas of the secondmodule intersects a second module longitudinal axis at a second angle;and the first and second angles are unequal; using the first and secondmodules to make measurements; and using the measurements to determinethe formation property.
 69. The method of claim 68, further comprisingusing the formation property to steer a drill bit.